This disclosure is directed to a process to convert polymeric packing materials into completed packing particles which are electrically conductive across the surface. More particularly, the present procedure accomplishes conversion of polymeric surfaces on packing materials into conductive surfaces at milder conditions and in shorter treatment times during the practice of the improved process of this disclosure. The polymeric packing material is foamed polymer (described below) formed into peanuts of random sizes and shapes. For many applications, the foam is relatively light weight, and a typical density is in the range of about 2 to 6 pounds per cubic foot of foamed material. When formed into peanuts including discs, spheres or other shapes loosely heaped in a packing container or carton, the effective or net density is typically even less by perhaps 20 to 40%, depending on peanut shape irregularity.
The irregular shape is an advantage as a packing material. There is a problem in handling polymeric packing material, as might arise on pouring loose particles into a packing crate. The random, sliding movement may build up static electrical charges on the surfaces of the poured particles, thereby causing light particles to stick together. Indeed, this can also cause static electricity to damage the item being shipped in the packing crate. While some items are not harmed by the static electrical charge, many products requiring protection by foamed packing material are very sensitive to electrical charge. This is especially the case for electronic equipment having integrated circuit components. Often, the peak voltage collected on a surface is surprisingly high, and will remain high for a long time. The static charge is stored until it has an opportunity to bleed across the accumulated peanuts to restore an electrically neutral condition to the previously charged area. While the total charge is extremely small, the extremely high resistance prevents current flow, causing the charge to linger on the surface for a long time and creates a risk of damage to the cargo protected by the packing material.
An important advantage of the present procedure is the ability to manufacture packing materials with a controlled surface conductivity, enabling charge bleed off. This ability to bleed off charge is obtained by forming a controllably conductive layer on packing particles. Assume that the packing material is randomly shaped peanuts having a range of sizes. The surface is modified without defeating the features of packing materials, namely, a sacrifical peanut of specified shape and density to enable the peanuts to protect cargo nested in the peanuts. In other words, the foamed packing material will not accumulate and hold a charge indefinitely and will not electrically cluster or clump, enabling the peanuts to pour freely from a container.
Perhaps a definition of various electrical conductivities will assist in identifying benefits of the present process. In general terms, a material which has an electrical conductivity of 10.sup.-15 /ohm cm is defined as an insulator. Any material which is less conductive than this can be treated as an insulator material. Where the conductivity is typically in the range of about 10.sup.-6 to about 10.sup.-9, an antistatic material is provided. An EMI shielding polymer typically will have a conductivity of about the range of 10.sup.-2 to about 10.sup.-6. A conductivity of about 1 is typical of silicon and the conductivity of graphite is about 10.sup.6. Conductive metals such as silver and copper typically have a conductivity of about 10.sup.9. The present procedure enables manufacture of polymeric packing materials which, subject to control of the process, can yield antistatic materials or EMI shielding materials.
Utilizing a feed stock which includes selected polymers, changes in the chemical structure can be made through the various steps of this disclosure for obtaining a conjugated double bond system in the polymer chain. The feed stock is polymers or co-polymers including polyethylene, polystyrene, polyolefin, and polyurethane in the foamed state. The product will be referred to generally as polymeric packing material.
The method of treatment disclosed has a great advantage in intermediate step fluorination which prepares the polymeric packing material (both polymer and copolymer system) for subsequent treatment. By contrast with the present disclosure, it is possible to expose selected polymers to a strong base such as ethylenediamine (EDA) for many hours at ambient temperature with little or no reaction. This prior process can be forced (by high temperature or pressure) to yield a polymer which is altered in conductivity. The present disclosure describes a dehydrohalogenation step which proceeds in quick order at ambient conditions. This disclosure sets forth a fluorine treatment enabling subsequent dehydrohalogenation at room temperature in short order, perhaps a few minutes. Absent the preliminary fluorine treatment, the only way to force the conversion through dehydrohalogenation is to utilize excessive temperatures of perhaps 100.degree. C. or higher and much longer contact intervals with the EDA to accomplish dehydrohalogenation.
It is one object of the present procedure to therefore provide a preliminary step to assist in conducting the dehydrohalogenation step in the presence of a strong base (EDA is typical) and to obtain controllable surface penetration and controllable conversion to selected ranges of electrical conductivity on polymeric packing material.
Other advantages of the present procedure will become more readily apparent upon an evaluation of the process described hereinbelow. Moreover, a product is manufactured as will be described. Various examples of the method of manufacture are also set forth. In like fashion, specific tests describing the electrical conductivity are also included.